SOHO FAQ: Sun Basics
Basic information and statistics about the Sun are
(Sun Fact Sheet)
(The Nine Planets: The Sun)
Hmmm, let's see. THE most important thing about the Sun? Can I list a few things?You probably already have the basic facts about the Sun, but here are some things that strike me as especially important and interesting for a 5th grader:
Let me answer both of those questions from the inside out. The core of the Sun is where the important nuclear reactions are taking place. It occupies the innermost quarter or so of the Sun's radius. At the very center of the core the temperature is 15 million Kelvin (or 27 million degrees Fahrenheit) and the density is 13 times greater than lead. Energy radiates from the core to about 80% of the way to the surface. Here, where the temperature is down to 4 million degrees F, the Sun's plasma becomes "convectively unstable." Energy rises above this point via convection, as in a boiling pot of water.
The Sun's visible surface (do NOT look at it!) is "only" about 5800 K (10,000 degrees F). Above the surface is a blanket of gases called the corona. The corona is up to 2 million deg. F, much hotter than the surface beneath. Why these gases get so hot has been a mystery for decades; SOHO's magnetic observations may help solve this mystery. So the short answer is: the Sun is 27,000,000 deg. F in the center and 10,000 deg. F on the surface.
Wow, I could write a book to answer this question (and some people already have)! But, I guess you don't want an entire book-length response, so I'll try to answer it succinctly.
The Sun is basically a great big ball of gas, mostly made up of hydrogen, the lightest element in the known universe. Near the center of the Sun, the weight of the overlying gas increases the pressure so much that nuclear "burning," or fusion, of hydrogen takes place, giving off a tremendous amount of energy. The temperature at the center of the Sun is about 15 million degrees Celsius!
The energy released in this way makes its way outwards towards the surface in the form of radiation (light) and gas motions (just as when you boil a pot of water on your stove, the hot water starts to bubble upwards). By the time the energy gets out to the surface of the Sun, it is spread out over a very large area, so the temperature of any particular location is much lower than it is at the center. In fact, the surface of the Sun is at a temperature of "only" about 5,800 degrees Kelvin.
An object at this temperature glows with a golden yellow color. Think of holding a metal bar in a hot fire: it will start to glow red as it heats up, and gradually it will turn yellow, then white as it gets hotter. The Sun's surface is just at the correct temperature to appear yellow in color. So this is why the Sun shines as it does.
I've condensed the results of many years of research and understanding into the above paragraphs. It is really amazing that we can understand in great detail what goes on inside a star almost 100 million miles away, not to mention much more distant stars and galaxies. The results now coming in from SOHO are helping us to understand even better how our Sun works, and how its output affects us here on Earth and sustains our life.
The Sun is heated by a process called nuclear fusion. Deep inside the Sun, particles combine to produce heavier particles and release energy. This process makes a LOT of energy. It happens because the Sun is huge and there is a lot of pressure at the center. Fusion does not normally happen on the Earth. See this website:http://www.astro.uva.nl/demo/sun/home.htm
Current astrophysical theory predicts that the Sun will become a red giant in about five billion (5,000,000,000) years.
We think that stars like the Sun shine for nine to ten billion years. How old is the Sun? It is about 4.5 billion years old, judging by the age of the Moon rocks. The red giant phase will occupy most of the last few hundred million years of the Sun's lifetime.
Good question. Even scientists can debate the answer to this one. Two leading answers are "yellowish-white" and "green." Our eyes perceive sunlight as being yellow or white -- but DO NOT LOOK AT the Sun to confirm this!
It is also correct to say it is "green," too, because the Sun's peak wavelength (500 nanometers, or 5000 Angstroms) is in the green part of the visible spectrum. Watch this site for further resources regarding this debate. Further discussion can be found at the Stanford SOLAR Center:http://solar-center.stanford.edu/FAQ/Qsuncolor.html
Two types of images are displayed on our web pages (and throughout the SOHO community) in "green." The C1 FeXIV coronagraph images are taken through a FeXIV filter in the neighborhood of 5302 Angstroms (530.2 nanometers, or well within the visible range). Because it is in the "green" part of the spectrum, the FeXIV solar emission line is traditionally known as the "green line." Hence, we show it in a green color table.
The 195 Angstrom EIT images are one of four wavelengths which EIT sees. The "E" in "EIT" means extreme ultraviolet. The human eye cannot see any radiation below about 4000 Angstroms, so we humans cannot see these EIT wavelengths. The EIT team assigned a unique color table to each of these wavelengths early in the program, but there is no intended correlation to actual "colors." (I asked that very question myself when I signed on to this project.)
The Earth is about 93 million miles, or 150 million kilometers, from the Sun. This distance varies slightly throughout the year, because the Earth's orbit is an ellipse, not a perfect circle. (The Earth's seasons are caused by the tilt of the planet, not the varying distance to the Sun. Many people do not realize this yet.)
SOHO is between the Earth and the Sun, 92 million miles from the latter and only about 1 million miles from the former. SOHO isn't that much closer to the Sun, so the spacecraft is in no danger of "burning up!"
The Sun, our star, is not simply a quiet ball of nuclear fire out in space. It bubbles and seethes with electromagnetic activity. Its surface gets speckled with sunspots and active regions. It occasionally spits out clouds of plasma and energetic particles.
All of this activity follows an 11-year cycle. Every 11 years, the number of sunspots, flares, and solar storms increases to a peak. Then, after a few years of high activity, the Sun will ramp down to a few years of low activity. This pattern is called the "sunspot cycle," the "solar cycle," or the "activity cycle." We have known about the cycle since the 19th century, because it was seen in sunspot records.
The Sun spends a few years at the peak of its cycle. This is known as "solar maximum." The few years during the less-active part of the cycle are called "solar minimum," or "quiet sun." Big solar flares and CMEs are more likely during solar maximum, but they can occur at any time.
The phrase "quiet sun" can also refer to a magnetically "quiet" part of the Sun's surface. SOHO's spectrographic teams often talk about observing "quiet sun" if their targets are free from filaments or active regions.
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